1. Field of the Invention
This invention relates generally to the formation of magnetic films for use in fabricating recording heads suitable for writing on high density magnetic media. In particular the invention teaches a plating method for the formation of an alloy of novel composition and particularly advantageous magnetic properties.
2. Description of the Related Art
Magnetic write heads must be capable of recording on magnetic media with recording densities that will approach 100 Gb/in2 by 2003. The high coercive force necessary to record on such media, coupled with the high resolution required by the narrow trackwidth and recording density, will necessitate the formation of write head material with high saturation magnetization and low coercivity. Since modern write head manufacturing techniques have turned to the thin film magnetic head as the structure of choice, a method must be found to form such thin films with a saturation magnetic moment preferably greater than 21 kG (kiloGauss) and low coercivity, preferably less than 13 Oe. Materials having these advantageous magnetic properties have already been studied extensively. Osaka et al. (U. S. Pat. No. 6,063,512) provide a magnetic film of low coercivity (a “soft” film) having a Co—Ni—Fe ternary alloy composition and trace amounts of S and formed by a method of constant current electrodeposition. The film so provided is reported to have a saturation magnetization, Bs, of between 1.5 T (Tesla) and 2.0 T (between 15 kG and 20 kG) and a coercivity of less than 50 Oe (Oersteds). Further, Osaka et al. (U.S. Pat. No. 6,120,918) provide a magnetic film of high magnetic moment and low coercivity having a Co—Fe—Ni ternary alloy composition with mixed BCC (body centered cubic) and FCC (face centered cubic) crystal structure. Said film has a saturation magnetization, Bs, of between 19 KG and 22 KG and a coercive force no greater than Hc=2.5 Oe. Although the soft film provided by Osaka has low coercivity, its saturation magnetization is marginal for the high density recording media envisioned. Other methods for forming magnetic films also suffer from the lack of sufficient magnetization. In this regard, Hasegawa (U. S. Pat. No. 6,124,047) provides a soft magnetic film of a Co-M-T-C composition with advantageous resistivity and magnetostriction properties but having saturation magnetization of approximately 14 kG (1.4T). Suzuki et al. (U. S. Pat. No. 5,935,403) provides a method for manufacturing a magnetic thin film in which colloidal particles of insulating material are suspended within a plating bath comprising Fe, Ni and Co ions. The thin film thereby formed is characterized by a saturation magnetization of between 1.5 T and 1.8 T, which is insufficient for the high density recording envisioned in the present invention.
Bozorth (“Ferromagnetism,” R. M. Bozorth, IEEE Press, New York, N.Y. 1978, p. 190) describes an Fe2Co alloy with a 24.3 kG maximum saturation moment. This alloy, however, is conventionally produced by bulk melting and high temperature thermal treatment, processes which are not suitable for magnetic write head formation. In addition, as noted by Yun, et al. (“Magnetic Properties of RF Diode Sputtered CoxFe100-x Alloy Thin Films,” IEEE Trans. On Magnetics, 32(5), 9/1996, p 4535) this alloy also has an unacceptably high coercivity for application to write heads.
The particular method of electrodeposition applied to the formation of magnetic films also plays a role in achieving their advantageous properties. In this respect, Asai et al. (U.S. Pat. No. 5,489,488) teach an electroplating process to form a soft magnetic multilayer film whose successive layers are formed by alternating the current direction within the electrolyte. Liao et al. (U.S. Pat. No. 4,756,816) teach an electroplating method using a low toxicity bath in which sodium saccharin acts as a stress relieving agent, boric acid acts as a pH buffer and dodecyl sodium sulfate acts as a surfactant to eliminate pitting.
An effective method to reduce film coercivity is by promoting grain refinement (smaller grain sizes). Grain refinement is generally achieved by enhancing nucleation or impeding grain growth during electrodeposition. As-deposited materials of mixed structure generally have smaller grain sizes because competition between structures promotes nucleation which, in turn, leads to more, but smaller, grains. The mixed FCC and BCC crystals of Co—Fe—Ni disclosed by Osaka et al. above is an example of the use of multiple structures to reduce grain growth. Multiple, co-existing structures can also be formed by the addition of minor amounts of elements such as Mo, Cr, W and Rh. FIG. 1 shows a phase diagram for a Co—Fe—Mo in which the Mo is present in approximately 5% by atomic weight. As can be seen, this small amount of Mo produces α, γ, and θ structures.
Another approach for reducing grain size is incorporating materials through use of a dispersed metal oxide. The oxide interrupts grain growth and thus enhances nucleation during electrodeposition. Oxides of Mo, W, Cr and Rh can be deposited from an aqueous solution under an anodic potential. FIG. 2 shows that MoO3 can be deposited from an aqueous solution containing M+++ ions at anodic potential greater than 0.2V and a pH less than 4. FIG. 2 also shows that that Mo can be oxidized to MoO2 at a slightly cathodic potential. The MoO2 can then be further oxidized to MoO3 at an anodic potential in an acidic environment.
Electroplating is an effective method for producing thin film magnetic alloys. Co, Fe, Ni, Mo, Cr, W and Rh can be readily co-deposited from an aqueous solution of their salts by use of a cathodic current. The alloy content can be adjusted by the solution concentration and current density. The more concentrated element in the solution generally produces the more concentrated element in the alloy. Higher current density favors the reduction of the element with the higher reduction potential.
Adjusting plating parameters can fine tune some of the mechanical and magnetic properties of the alloy film. For example, the addition of saccharin is known to reduce stress within the film (see Liao et al., cited above.) Pulse and pulse reversal plating provides two potential advantages over direct current plating. One such advantage is the reduction of grain size by grain growth interruption with corresponding lowering of coercivity. Another advantage is improved micro-uniformity. The anodic period of the current allows the metal ion to be replenished, producing a uniformity of metal concentration across the topography of the film. This is particularly advantageous in plating applications wherein the film is to be deposited in trenches with high aspect ratios, such as is the case when plating upper pole pieces of magnetic write head elements.